Um... I'm gonna have to invoke "pics or it didn't happen" on your xbow>mech infantry argument, hewhok.
I believe you meant a city with A crossbowMAN in it, not crossbowMEN (1UpT, remember?). What's stopping you from wiping the floor with any xbows chilling outside the city? Then, the enemy can only plink off around 2 hp at most with their last xbow and city bombardment. (I've regularly seen lower-tech ranged units do 0 damage to even normal infantry that had at least somewhat of a defensive position)
Even a xbow at a chokepoint (on a hill, with a forest) gets absolutely mashed by attacks from pre-gunpowder units. Mech infantry mash, puree, and consume. Scramble's advice was for the early gunpowder era, where muskets can still destroy xbows as long as you don't leave them out in the open to get shot. Learn to adapt; it's not that hard.
Also, how the hell does a transition period not make sense? (from both a gameplay and realism level, to boot?) Do you want to go directly from medieval to modern? That would completely screw up the balance of the game. As it stands, it makes PERFECT sense that units from one era can somewhat (and I stress "somewhat") compete against units from the next era IF USED PROPERLY. If used improperly, xbows die very easily. So do muskets. So do mech infantry, for that matter, but that's against other modern units. Against older units, especially medieval ones, the "attack" command works just fine.
One last question: did you send a lone mech infantry to take out an entire civ or something? If you did, then yes, it may have lost to an entire army of lower-tech units. But, if you support it with more mech infantry, some artillery, planes, battleships, etc. (you know, all of those strong units you can build) you can win against even modern armies as long as you use a modicum of strategy.